1. Field of the Invention
The present invention relates to devices used to pull connectorized optical fiber cables through cable ducts, and more particularly, to such devices for use with what is known as loose buffer tube optical fiber cable.
2. Description of the Prior Art
The use of optical fibers is becoming prevalent in the telecommunications and data communications industries, and new building structures are being provided with cable ducts for the subsequent installation of optical fiber cables. Additionally, the telephone companies have found that the space in existing duct systems can be more effectively utilized by the use of optical fiber cable because of its small outer diameter, which is typically less than 0.90 inch.
A typical 4-inch duct has installed therein sub-ducts such as, for example, three 11/4-inch sub-ducts, or four 1-inch subducts, through which optical fiber cable is to be installed.
A typical optical fiber cable is represented in FIG. 1, where there is shown a loose buffer tube type cable 2. The cable is formed about a central strength member 4 which may be formed of either a single steel or a dielectric member or a plurality of twisted members. Disposed about the central strength member 4 is usually a plastic jacketing material 6. Buffer tubes 8 carrying optical fibers 10 are wrapped around the central strength member in a helical or reverse helical manner. On occasion, filler tubes, such as a tube 12 are used to take up space when the cable capacity does not require the use of all buffer tubes. A binder tape or thread 14 is helically wrapped around the buffer tubes for maintaining them in the proper orientation, and a first inner polyethylene jacket 16 is extruded about the binder tape. A ripcord 18 is provided to assist in removing the inner jacket 16. A strength and protective member in the form of a corrugated steel armor layer 20 may be provided for additional protection against possible rodent damage. The armor 20 is usually coated with a thin layer of plastic material. A second ripcord 22 is used to assist in removal of the armor 20. A final polyethylene outer jacket 24 is provided for additional cable protection.
The above structure is a typical loose buffer tube cable construction used in many optical fiber installations at the present time.
Due to the extremely small size of the optical fiber and the difficulty of making fiber splices in the field, it has become common to provide connectorized optical fiber cables wherein connectors are provided at the ends of each buffer tube 8 for connecting the fibers within the buffer tube to the fibers of another cable. Connectorized cables are produced by a number of manufacturers including Alcatel Cable Systems and AT&T.
Due to the fragile nature of optical fibers, pulling cables through pre-installed ducts is a difficult and tedious task, which task is exacerbated when the cable is connectorized and a plurality of connectors at the ends of buffer tubes must also be pulled through the ducts. Invariably, the cable connectors would be damaged during installation. Pulling eye assemblies were devised to facilitate the installation of cables through ducts. The pulling eyes provided a means for gripping the cable and, in particular, the strength components of the cable, so that stresses would not be exerted on the optical fibers while the cable was pulled through the duct system. With the advent of the connectorized optical fiber cables, the pulling eyes assumed an additional task of protecting the cable connectors during installation.
Most manufacturers of connectorized cable provide some form of pulling eye. In most pulling eyes, a braided metallic hose was used to attach a nose piece to a cable clamping device, with the pulling tension being exerted on the braided metal hose, as opposed to the cable components. The protected interior of the braided metal hose provided a secure chamber in which the cable connectors could be protected.
Due to the small size of the cable ducts, the cable diameter had to be maintained at a minimum, as did the diameter of the pulling eye assembly. Most existing pulling eyes have too large a diameter and therefore require larger duct sizes. In addition, the pulling eye had to be of such size that it could pass through bends having a predetermined minimum radius. A standard size requirement for pulling eyes is that a cable with an outer diameter of 0.750 inch must pass through a 1-inch sub-duct with a 24-inch bend radius.
Another requirement that must be met during the installation of a non-dielectric connectorized cable through existing ductwork is that electrical continuity must be maintained throughout the length of the optical fiber cable system, so that the cable can be grounded to drain any electrical charge that may build up on the cable, either as a result of static electricity or inadvertent contact with an electrical source. Thus, grounding of the steel armor layer is essential for safety purposes.